The flexographic industry has developed over the years from hand carved rubber plates with no expectations of register control between colors, to use of a photopolymer plate 100, shown in FIG. 1, supported by a polymer sheet 101 to provide dimensional stability and allow registration between colors.
Flexographic printing was the least of the print processes in terms of capabilities and the lowest cost compared to the traditional offset, letterpress and rotogravure printing processes used for package printing. Since the introduction of the photopolymer plate making systems, the growth in use of flexography has been significant, becoming the largest printing process used in packaging, and in some regions, like North America gaining over 80 percent of market share.
Throughout the development process, registration systems have steadily improved as the flexographic printing process has improved. The flexographic plate is selectively exposed to ultra violet light and unwanted areas washed away leaving a raised printing surface 103, shown in FIG. 2. Exposed photopolymer 102 with a lower floor level 104 that does not print, is separated by a displacement called the relief height 105, hence flexography is a relief printing process. The vast majority of the flexographic plates manufactured today are in sheet format, and are attached to a printing cylinder or sleeve 107, shown in FIG. 3, in register, using highly engineered double sided sticky back tape 106 with specific adhesion and compressibility properties for best print quality or productivity or both.
The plates 110, shown in FIG. 4, all have some form of register mark 111 and 112 on each side of the plate to allow easier plate to plate registration so that minimal time is spent in registration on press setup and optimum image quality is achieved. Over the years the types of register marks have progressed as mounting systems and methods have advanced. Originally the register marks 111 and 112 shown in FIG. 4, were large cross hairs for register on press. Placing the plates was a highly skilled process and often resulted in mis-register for at least one of the colors.
To enable greater productivity and accuracy, video mounting systems were developed, shown in FIG. 5. Two or more cameras 121, 123, were positioned on a frame 120, relative to a cylinder, for mounting the plate cylinder or sleeve 107, with focal points 122 and 124 on the surface of the cylinder or sleeve. The cameras were adjusted laterally 125 for the first plate and aligned with the plate cylinder or sleeve 107. Mounting tape is applied, and the plate is positioned above the plate cylinder or sleeve, shown in FIG. 6, with minor adjustments 126 to match the register marker 111 and 112, before fixing the cameras 121 and 123 in place. The plate is then affixed to the mounting tape 106 and the plate is imaged. Additional plates are mounted without adjustment of the camera position, as shown in FIG. 7, for accurate plate to plate location.
As demand for flexographic printing process grows, it is moving to process printing, that is, building images and colors out of four (CMYK), six, or seven process colors. It is important for process printing that the colors are in accurate registration to each other. With the printed dots being as small as 10-20 microns, any shift in registration can cause color shifts, image errors, or interference patterns with a negative impact on the final image. This has driven the industry to smaller register marks, shown in FIG. 8, with the large manual cross hairs 130, moving to smaller cross hairs 131 for video mounting, and then microdots 132.
As applications for functional printing develop for very small lines and circuits as small as five microns in width, and the need for accurate layer to layer registration, on printing register, and the mounting of the plates accurately on the print cylinders and sleeves increases, there is a further need for improvement in microdots. The current state of the art is to place 2 or more microdots on each of the plates, 115, 116, shown in FIG. 9, and use video mounting systems to locate and position the plate manually or automatically, shown in FIG. 10, and then fix the plates in place on engineered double sided mounting tape on the sleeve or cylinder.
Current microdots are typically 200-250 microns in size. A recent publication, U.S. Publication No. 2011/0265676, describes a registration system employing a scattered microdot pattern with each dot about 200 microns. Such large dots are objectionable when visible in the printed product. Smaller registration features are desired to ensure invisibility. In the printing of functional materials, such as electronic circuits, component sizes of five microns or less are desirable. When printing multiple layers, registration accuracy must be improved.
Traditionally the size of the microdots in flexo is limited to the size of a separate stand alone dot made of a group of pixels that can be consistently and independently formed on the plate. These are described as the minimum isolated dot size. In the majority of the flexoplate market this is presently between 120 and 250 microns.
Traditional digital flexo imaging technology uses Gaussian lasers, 140, shown in FIG. 11, versus the proposed technology using SQUAREspot imaging 141. The traditional Gaussian imaging produces an error in imaging 142, shown in FIG. 12, that limits the capabilities of imaging and image transfer, unlike the SQUAREspot imaging 143. This invention uses the exact reproduction of the original digital data 150, shown in FIG. 13A, on the thermal imaging layer 151, shown in FIG. 13B, and to the final plate 152, shown in FIG. 13C, as shown in the photograph in 153 in FIG. 14.